The Avengers are gathered together in Avengers Mansion, licking their wounds after their not-quite victory with Thanos. True they've driven Thanos back for now and secured the Infinity Stones, but it came with a heavy price, including the loss of Vision, Captain America, and the impostor Bruce Banner/Hulk, who it turned out was a spy for Thanos this whole time. Where the real Banner is located, is anyone's guess...
Someone flips on a tv.
"...take something truly remarkable to take the headlines away from the recent attempted invasion of earth by the alien warlord Thanos, but researchers at NYU have made a remarkable discovery. It seems that the numbered of powered individuals across the globe have been increasing at a rapid rate. Previously, most of these beings were believed to be "Inhumans", referring to humans who received powers as a result of contact with alien materials. But research now shows that most powered individuals are actually born with a gene that makes them different than humans. These individuals don't exhibit powers right away, but usually emerge during their adolescence, often as a result of trauma or stress. Researchers claim this is a form of genetic mutation..."
Scarlet Witch: "Mutation?"
Iron Man: "I mean that makes sense if you think about it. Evolution has been stalled for hundreds of thousands of years. We were due for a leap forward. In fact honstly Wanda, we should have you do a DNA test. It's entirely possible you're one of these..."
S. Witch: gives him a severe look "Mutants?"
Iron Man: "I was going to say differently powered? I..."
Suddenly Tony is interrupted by a huge crack of thunder and lightning with lights up the sky through their large bay window. The sky then turns immediately dark and thunder continues to rumble outside.
Iron Man: "Thor?"
Thor: "This is not me."
Iron Man: "Can you make it stop?"
Thor: Closes his eyes in concentration. "I... cannot." He looks surprised.

The sound of compressed air behind them off panel. Suddenly there are sparks from a power cable on the wall. The room goes dark and a back up generator kicks on, bathing them in dim/reddish light. That compressed air sound again, the sound of punches, and when they turn around, the two SHIELD Agents who were guarding the door are now lying there unconscious. Music begins to build, into a theme that sounds familiar but not for the MCU.

The elevator doors open. Into the room step Cyclops, Storm, Wolverine, Jean Grey, and Colossus. The compressed air sound again, and Nightcrawler appears behind the Avengers. The music coalesces into the X-Men theme song.

Cyclops puts his hand up to his visor. "We're the X-Men," he announces "and we've come for her." Looking at Scarlet Witch. Snkkkkkkt.